1. Field of the Invention
The present invention relates to a waveguide type signal terminator and signal attenuator which can be used as an element for wireless communication systems and measuring apparatuses. More particularly, the present invention relates to a waveguide type signal attenuator for attenuating an input signal to a desired state and a waveguide type signal terminator for making an input signal be vanished in which a resistor sheet acting for signal attenuation or termination is inserted into the central area of the waveguide, along which a traveling electromagnetic wave has the strongest intensity, of the signal attenuator and the signal terminator.
2. Description of Prior Art
Recently, in order to realize a wireless communication of very high speed and massive capacity of data traffic, many tries for a wireless communication have been made by means of transmitter/receiver in a band of millimeter wavelength. In wireless communication systems used in the band of millimeter wavelength, such systems using small loss waveguides are widely used and waveguides are also broadly employed by a variety kind of elements and measuring apparatuses. In such wireless communication systems and measuring apparatuses, there are many cases that signal attenuation or signal termination is required. In those cases, needed are an attenuator which makes an input signal be attenuated by a certain ratio into and/or a signal terminator which makes an input signal be completely terminated. An example for a prior signal attenuator and a prior signal terminator is illustrated in FIGS. 1A to 2B.
FIGS. 1A and 1B are perspective views which show an example of a prior waveguide type terminator, showing one state that an absorbing body is installed and another state that the absorbing body is detached.
According to the structure of the prior waveguide type signal terminator 1 as shown in drawings, a waveguide 12 of an elongated cavity line structure of which an exit is closed is formed in a lower conductive plate 10 and the open top of the waveguide 12 is covered with an upper conductive plate 30. In addition, in the waveguide 12, an absorbing body 20 is installed. In order to secure a function of the signal terminator 1 which attenuates an input signal and terminates it finally, the output port of the waveguide 12 is closed. In the front portion of the absorbing body 20, a V-groove 22 is formed with its vertex orienting to the rear of the absorbing body 20. The absorbing body 20 is usually made from raw materials of a ceramic system.
The signal terminator 1 with such a structure as above is required to have a function of signal absorbing so that an input signal entering a signal input port 14 can be completely disappeared without causing any reflection of the input signal at the end portion of the signal terminator 1. In order to meet this requirement of no signal reflection in the signal terminator, there should be given an impedance match between a waveguide portion inserted with the absorbing body 20 and its neighboring waveguide portion. The impedance match can be obtained by making the length d1 of V-groove 22 be equal to the wavelength-in-waveguide μg of the input signal. Besides, a signal absorbing rate by the absorbing body 20 can be adjusted by varying the length d2 of the rear portion behind the V-groove 22 of the absorbing body 20. The signal absorbing ratio increases in proportion to the length d2 of the rear portion behind the V-groove 22, but it is saturated over a threshold value.
To obtain a good characteristic of the signal terminator 1 as such, accurate works are needed in designing and manufacturing the absorbing body 20, particularly as to the length d1 of V-groove and the length d2 of the rear portion. There are some difficulties in manufacturing the prior signal terminator 1 which requires accurate manufacturing works, and thus such requirement pushes up the manufacturing cost.
Meanwhile, FIGS. 2A and 2B are perspective views which illustrate a prior waveguide type signal attenuator, where FIG. 2A shows a state that a resistor card is installed and FIG. 2B shows a state that the resistor card is detached.
According to the prior waveguide type signal attenuator 3 as shown in FIGS. 2A and 2B, a waveguide 52 of an elongated cavity of which both ends for input and output are opened is formed in a lower conductive plate 50 and the open top of the waveguide 52 is covered with an upper conductive plate 70. In addition, in the waveguide 52, a resistor card 60 is installed. The resistor card 60 resembles a semi elliptical shape of which height becomes smoothly lower from center to both ends. The resistor card 60 having the semi elliptical shape gives an impedance match between two sections, where one section is installed with the resistor card 60 and the other section is not, of the waveguide 52 so that it can effectively suppress the signal reflection. That is, an input signal inputted to a signal input port 54 of the waveguide 52 is attenuated by the resistor card 60 and the attenuated input signal is outputted through a signal output port 56.
An intensity of electric field of the input signal has the maximum value along the center area of the waveguide 52, that is, along the bisecting line of the width of the waveguide 52 and thus the resistor card 60 is located along the bisecting line to obtain the best impedance match and the maximum signal attenuation ratio. For the installation of the resistor card 60, the waveguide 52 is formed with an insertion groove 58 having the same thickness with the resistor card 60 and the resistor card 60 is inserted into and fixed to the insertion groove 58.
A signal attenuation ratio is determined in accordance with a projected area of the resistor card 60 which is inserted into the waveguide 52. In order to obtain a required signal attenuation ratio, an insertion depth of the resistor card 60 into the insertion groove 58 should be suitably determined to adjust the projected area of the resistor card 60 which projects into the waveguide 60. However, the signal attenuator 3 having such a structure has some defects that it is difficult to determine an accurate position at which the resistor card 60 is installed to obtain a precise signal attenuation ratio and is also not easy to form the insertion groove 58 for the resistor card.